The main function of lubricating oils is to reduce the friction between parts that move relative to one another, by the formation of a fluid surface film, as well as to protect the parts against corrosion, and to assist in sealing and in the transfer of heat between the contacting surfaces. Usually these lubricants are prepared from a mixture of mineral or synthetic oils with various additives, the oils of mineral origin being those obtained by processes of distillation and refining of petroleum and the synthetic oils being those obtained by a process of synthesis using raw material different from the former.
The oils of mineral origin are not easily degraded or absorbed by the environment, which has in recent years aroused special interest in the advantages offered by substances derived from oils of vegetable origin, such as biodegradability and lower toxicity. However, these oils possess low thermal-oxidation and hydrolytic stability and in order to improve these properties, the fatty acids that make up the vegetable oils must undergo modifications in the carbon chain.
Estolides are derivatives of vegetable oils that have been shown to offer new promise for application as lubricants, due principally to their excellent properties at low temperatures, the pour point being one of the best indicators of such properties. The pour point is the lowest temperature at which the oil still flows freely under the action of gravity, after cooling in standardized conditions, and is extremely important when the lubricant must meet requirements of low-temperature viscosity.
Estolide is a generic name for linear oligomers of polyesters of fatty acids, in which the hydroxyl of a hydroxylated fatty acid is esterified by the carboxyl of another molecule of fatty acid.
U.S. Pat. No. 5,380,894 describes a process for the synthesis of estolides by the reaction between one or more unsaturated fatty acids in the presence of a catalyst, usually clay and water, in the temperature range from 230° C. to 250° C. and at initial pressure in the range from 200 kPa (30 psi) to 415 kPa (60 psi). The estolides thus produced can be used as lubricants, greases, plasticizers and printing inks, as well as in cosmetics.
U.S. Pat. No. 6,018,063 relates to a family of estolides derived from oleic acid, which are characterized by superior properties when used as lubricants. Among these properties, we may mention in particular: their high viscosity index, which avoids the use of additives that might cause problems connected with stability; their high oxidation stability compared with vegetable oils or fluids derived therefrom; and their low pour point, allowing them to be used as lubricants even at low temperatures.
In the cases described above, the estolide produced has double bonds in its structure. It is known, however, that its greater chain size permits better electronic distribution of the charges of the molecule, stabilizing the double bonds. Furthermore, the molecule of fatty acid added to the structure of the original ester tends to behave like a branching, generating a molecule with format similar to that of a ball of wool, making it difficult for oxygen to gain access to the double bonds of the structure, and consequently increasing the oxidation stability.
The synthesis of estolides from fatty acids gives a product with a large quantity of residual free fatty acids and consequently high total acid number (TAN).
In the specialized literature, the processes used for the removal of residual fatty acids involve vacuum distillation, in vertical distillation apparatus, at temperatures of approximately 200° C. and pressures of the order of 10 Pa (0.1 mbar). However, one of the problems encountered when using said purification process is the formation of epoxides or shorter-chain carboxylic acids, resulting from the oxidation of the double bonds present in the free fatty acids, which are highly unstable.
Isbell et al., in their article “Purification of meadowfoam monoestolide from polyestolide” (Industrial Crops and Products, Vol. 15, 145-154 (2002)), describe other processes for purification of estolides, including molecular distillation. The purpose of this is to separate the mono- and polyestolides, for subsequent use of the monoestolides in the formulation of cosmetics, as they possess suitable coloration for said use.
Therefore, at present no purification process for estolides is available in the prior art that involves simple and economical systems for the removal of residual fatty acids from estolides, such as the process described below.